I'm assuming your laptop is running Windows
with the projector connected to your laptop
right click on the desktop
click on Properties
click on the Settings tab
you should see a gray box with two smaller '1' and '2' boxes
they represent the different displays available for output
the '1' is the LCD screen on the laptop, and the '2' represents the video output such as another monitor or projector
right click on the '2' and click Attached
click on Apply at the bottom and you should see the projector display your desktop wallpaper
in PowerPoint click on Slide Show at the top
then click on Setup Show
you should see a drop box under Multiple Monitors, choose the secondary monitor option
you should be good to go from there